1. Field of the Invention
The present invention relates to a laser-diode-pumped solid-state laser having a solid-state laser crystal that is pumped by a semiconductor laser in the form of a laser diode, and more particularly to a laser-diode-pumped solid-state laser having a wavelength conversion capability.
2. Description of the Prior Art
Japanese Unexamined Patent Publication No. 62(1987)-189783, for example, discloses a laser-diode-pumped solid-state laser having a solid-state laser crystal doped with a rare-earth element such as neodymium (Nd), the solid-state laser crystal being pumped by a semiconductor laser. One widely used material of the solid-state laser crystal is an ion crystal of a paramagnetic substance such as YVO.sub.4, YAG(Y.sub.3 Al.sub.5 O.sub.12), or the like that is doped with Nd.
According to one conventional design involving such a laser-diode-pumped solid-state laser, the resonator houses a bulk single crystal of a nonlinear optical material for converting the wave of a solid-laser-oscillated beam into a second harmonic thereby to produce a laser beam of a shorter wavelength, as disclosed in U.S. Pat. No. 5,124,999 and Laser Research, Vol. 18, No. 8 (1990), pages 94-99, for example.
The laser-diode-pumped solid-state laser should preferably be oscillated in a single longitudinal mode in order to suppress output fluctuations due to a longitudinal mode competition. In applications using laser-diode-pumped solid-state lasers, there is a demand for an increase in the pumping intensity of the semiconductor laser for a higher output power.
Laser Research, Vol. 18, No. 8 (1990), page 646, has reported that the wave of an oscillated beam from Nd:YVO.sub.4 (with an Nd ion concentration of 1.1 at %) is converted in wavelength into a second harmonic by a KTP crystal disposed in a resonator, the second harmonic having an output power of 12.8 mW in a substantially single longitudinal mode in response to an input power of 760 mW from a semiconductor laser. It has also been reported in the preprint of the symposium Laser/Atomic Oscillator And Ultimate Light Quantum Engineering (sponsored by Applied Physics Society and Quantum Electronics Society), E-2 (1990), page 56 that Nd:YVO.sub.4 (with an Nd ion concentration of 2.02 at %) and a KTP crystal having a thickness of 7.0 mm are used to obtain a second harmonic output power of 9.1 mW when the input power from a semiconductor laser is 740 mW.
With the conventional laser-diode-pumped solid-state lasers, however, it is necessary to control the resonator length strictly with a piezoelectric element or the like so as to obtain an output power in the single longitudinal mode. Even if the resonator length slightly varies on account of a temperature change or a mechanical vibration, the oscillation in the single longitudinal mode easily changes to an oscillation in a multi-longitudinal mode, resulting in mode completion noise. Therefore, the conventional laser-diode-pumped solid-state lasers have not been put to practical use because of such a problem.
According to IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 26, No. 9 (1990), page 1457, the condition to be met for achieving an oscillation in a single longitudinal mode is given by: EQU Pin/Pth.ltoreq.Wth (1)
where Pin is the pumping intensity of a semiconductor laser, Pth is the oscillation threshold value of a solid-state laser, and Wth is the threshold value of the relative pumping intensity of an oscillation in a multi-longitudinal mode. Under this condition, the oscillation in the single longitudinal mode is stable even when the resonator length somewhat varies due to a temperature change or a mechanical vibration. For such a stable and practical oscillation in the single longitudinal mode, the pumping intensity of the semiconductor laser has to be set to a level lower than a certain upper limit. Since, however, the upper limit for the pumping intensity in the conventional laser-diode-pumped solid-state laser is relatively low, it has been difficult for the laser-diode-pumped solid-state laser to meet both requirements for a stable oscillation in a single longitudinal mode and a higher output power. The conventional difficulty in meeting those requirements will be described below.
The threshold value Wth in the equation (1) above increases with the pumping optical absorption coefficient .alpha. of a solid-state laser crystal. Inasmuch as the optical absorption coefficient .alpha. is proportional to the Nd ion concentration, it is effective to increase the Nd ion concentration for increasing the threshold value Wth to achieve an increase in an upper-limit pumping intensity Pin for a stable oscillation in the single longitudinal mode.
If the Nd ion concentration is increased, however, the oscillation threshold value may often increase due to concentration quenching, with the result that the solid-state laser crystal may either fail to oscillate or may produce an extremely small output power upon oscillation. A resonator loss L of a solid-state laser crystal and an oscillated beam intensity Pc of the solid-state laser crystal are related to each other as follows: EQU Pc.varies.(Pin-Pth)/L (2)
The fluorescence lifetime .tau. of the solid-state laser crystal and the cross-sectional area .sigma. for stimulated emission are related to the oscillation threshold value Pth as follows: EQU Pth.varies.L/(.sigma..tau.) (3)
The fluorescence lifetime .DELTA. and the fluorescence intensity (which is proportional to .sigma..tau.) are related to the Nd ion concentration in Nd:YVO.sub.4 as shown in FIG. 3 of the accompanying drawings, which is cited from OPTRONICS (1990), No. 12, page 60. It is known that the illustrated relationship holds true for other ion crystals of paramagnetic substances. As shown in FIG. 3, if the Nd ion concentration exceeds 1 at % in the ion crystals of paramagnetic substances, the value of .sigma..tau. sharply drops due to concentration quenching. Then, as can be understood from the equation (3) above, the oscillation threshold value Pth increases to a value higher than, or very close to, the pumping intensity Pin. As a result, the oscillated beam intensity Pc may become zero or extremely small, as can be seen from the equation (2).
Because of such a phenomenon, the Nd ion concentration in the ion crystals of paramagnetic substances should optimally be in the order of 1 at % as described in OPTRONICS and Laser Research, Vol. 18, No. 8 (1990), page 646, as referred to above. A maximum possible value of 3 at % for the Nd ion concentration is only reported in IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 26, No. 9 (1990), page 1457.
Heretofore, the Nd ion concentration has been set to a value less than 3 at %, usually of about 1 at %. Consequently, the optical absorption coefficient .alpha. is relatively small, and the threshold value Wth is of a small value of at most 7. Therefore, as can be seen from the equation (1), the pumping intensity Pin must be reduced to enable a solid-state laser to oscillate in a single longitudinal mode. The oscillated beam intensity Pc of the beam from the solid-state laser that oscillates in the single longitudinal mode is also relatively low.
The technical limitation that the Nd ion concentration is preferably of about 1 at % or at most 3 at % has been widely accepted with respect to all laser-diode-pumped solid-state lasers which employ ion crystals of paramagnetic substances. Accordingly, it has been difficult for the laser-diode-pumped solid-state lasers to meet both requirements for a stable oscillation in a single longitudinal mode and a higher output power in applications to obtain short-wavelength laser beams through wavelength conversion.